The present invention relates to a method of preparing a rigid foam made of polyurethane-modified polyisocyanurate.
There is increasing demand for improvements in flame retardancy of a rigid polyurethane foam. As a procedure to deal therewith, there is a method in which an isocyanate is used in excess to form a rigid foam containing an isocyanurate compound in a large amount. However, as the amount of the isocyanurate compound is increased, the resulting foam becomes more brittle than usual polyurethane foam, and when used for producing a composite material having a surface material such as iron plate, the foam suffers from the problem of poor adhesion thereof to the surface material.
Conventional procedures taken for solving this problem involve increasing the reaction temperature; decreasing the content of water in compounded polyols while increasing a blowing agent consisting of a low-boiling compound; increasing a catalyst for producing an isocyanurate; or using polyols having a relatively high molecular weight. However, because of limitations such as other physical properties required for the foam and conditions for producing the foam, there was the problem that satisfactory adhesion could not be obtained particularly when e.g. polyester polyols are mainly used as the active hydrogen compound.
Polyols generally having a hydroxy value of at least 150 mg KOH/g, particularly at least 250 mg KOH/g, are used to produce a rigid foam having high compression strength and excellent dimensional stability. When a large amount of polyols having a hydroxy value of at most 100 mg KOH/g are used for the rigid foam having excellent thermal insulation performance and a high ratio of closed cell, there is the problem of poor dimensional stability, high shrinkage, etc. On the other hand, a large amount of polyols having a low hydroxy value can be used for the foam having a low ratio of closed cell or for the foam having high density without any problem of dimensional stability, but thermal insulation performance is deteriorated. Further, when high-molecular-weight polyols having a hydroxy value of at most 100 mg KOH/g are used partially for improvements in adhesion strength, etc., they tend to be separated owing to their poor compatibility with the polyols for a general rigid foam.
An object of the present invention is to produce a rigid foam which is excellent in compression strength, dimensional stability, flame retardancy and adhesion.
As a result of extensive study for achieving this object, the present inventors found that a polyester and/or a polyether polyol having a hydroxy value of at most 100 mg KOH/g is previously reacted with a polymeric MDI to form a prepolymer, and then this prepolymer is reacted with a formulated polyol to give a rigid foam excellent in compression strength, dimensional stability and flame retardancy, and also that when the amount of the polyol in the prepolymer is at least 5% by weight, the adhesion strength between the foam and a surface material is increased, and the present invention was thereby completed. The adhesion strength is increased with an increasing weight amount of the polyol in the prepolymer, but use of 30% by weight or more polyol in the prepolymer is not preferable because of a little increase in adhesion strength and the problem of poor dimensional stability, high shrinkage, etc.
The present invention relates to a method of preparing a polyurethane-modified polyisocyanurate foam, comprising reacting an active hydrogen compound having at least two functionalities with a polyisocyanate compound in the presence of a catalyst and a blowing agent comprising water alone or a mixture of water and a low-boiling compound, wherein:
(1) the polyisocyanate compound is a prepolymer obtained by reacting a polymeric MDI with 5 to 30% by weight, based on the polymeric MDI, of a polyether polyol and/or polyester polyol having a hydroxy value of at most 100 mg KOH/g, and
(2) the number of isocyanate groups in the polyisocyanate compound is at least 1.5 times by mole as large as the number of active hydrogen atoms in the active hydrogen compound and water.
The polyisocyanate compound used in the present invention is a prepolymer obtained by reacting a polymeric MDI with a polyether polyol and/or polyester polyol having a hydroxy value of at most 100 mg KOH/g. The polymeric MDI is generally a mixture of diphenyl methane diisocyanate and polymethylene polyphenyl poly-isocyanate. The content of isocyanate groups in the polyisocyanate compound is generally from 28 to 33% by weight, particularly from 30 to 32% by weight. The hydroxy value of the polyether polyol and/or polyester polyol may be, for example, at most 100 mg KOH/g, particularly from 23 to 80 mg KOH/g. The amount of the polyether polyol and/or polyester polyol is from 5 to 30% by weight, particularly from 5 to 20% by weight, based on the polymeric MDI.
The polyether polyol used to make the prepolymer includes hydroxyl group-containing compounds such as ethylene glycol, propylene glycol, diethylene glycol, glycerin, trimethylol propane, pentaerythritol, sorbitol and sucrose; and hydroxyl group-containing compounds having an alkylene oxide such as ethylene oxide or propylene oxide added to an amino group-containing compound such as diaminotoluene.
The polyester polyol used to make the prepolymer includes polyester polyols produced by a known method using at least one compound selected from ethylene glycol, propylene glycol, diethylene glycol, glycerin, trimethylol propane, pentaerythritol and sorbitol, and at least one compound containing at least two carboxyl groups, such as malonic acid, maleic acid, succinic acid, adipic acid, tartaric acid, pimelic acid, sebacic acid, oxalic acid, phthalic acid, terephthalic acid, trimellitic acid and polycarboxylic acid.
Examples of the active hydrogen compound having at least two functionalities, which is reacted with the polyisocyanate compound, include hydroxy group-containing compounds such as ethylene glycol, propylene glycol, diethylene glycol, glycerin, trimethylol propane, pentaerythritol, sorbitol and sucrose; amino group- and hydroxy group-containing compounds such as triethanolamine and diethanolamine; amino group-containing compounds such as ethylene diamine and diaminotoluene; and polyether polyols having at least two hydroxy groups in the molecule having an alkylene oxide such as ethylene oxide or propylene oxide added to e.g. a Mannich base compound formed by reacting phenol or its derivative, an alkanol amine and formaldehyde.
Further, examples of the active hydrogen compound include polyester polyols produced in a known method by using at least one compound selected from ethylene glycol, propylene glycol, diethylene glycol, glycerin, trimethylol propane, penta-erythritol and sorbitol, and at least one compound containing at least two carboxyl groups, such as malonic acid, maleic acid, succinic acid, adipic acid, tartaric acid, pimelic acid, sebacic acid, oxalic acid, phthalic acid, terephthalic acid, trimellitic acid and polycarboxylic acid. Further, polyester polyols manufactured by an ester exchange reaction between a high-molecular polyalkylene terephthalate polymer and a low-molecular diol such as ethylene glycol, propylene glycol, diethylene glycol, glycerin and trimethylol propane are also effective.
In the method of the present invention, a catalyst effective for conversion into isocyanurate and a catalyst effective for conversion into urethane, known as a catalyst in the chemistry of urethane, are used in order that isocyanate groups are used in excess over active hydrogen atoms (i.e. hydrogen atoms reactive with the isocyanate) to form an isocyanurate compound. The catalyst effective for conversion into isocyanurate includes e.g. organometallic compounds such as potassium acetate and potassium octanoate; quaternary ammonium salts such as DABCO TMR; and triazine compounds such as POLYCAT 41. The catalyst effective for conversion into urethane includes e.g. tertiary amines such as N,N-dimethylcyclohexylamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl ethylene diamine, bis(N,N-dimethylaminoethyl) ether and pentamethyl diethylene triamine; and organometallic compounds such as dibutyltin dilaurate and lead octylate.
Water alone is used or water and a low-boiling compound in combination are used as the blowing agent. The low-boiling compound includes hydrocarbons such as isomers of butane, pentane and hexane; and low-boiling fluorine-containing compounds such as HFC-245, HFC-365 and HFC-134a, and these are used alone or in combination.
Additives such as surfactants (foam regulators), for example silicone-based foam stabilizers and flame-retardants can arbitrarily be used as the aids.
It is preferable that among the physical properties of the polyurethane-modified polyisocyanurate foam produced according to the present invention, a ratio of closed cell is at least 70%, and a density is at most 70 kg/m3.
The rigid foam containing a large amount of the isocyanurate compound, which is obtained by the present invention, is excellent in the adhesion to a surface material such as an iron plate and useful as a thermal insulation panel applied to building materials, etc.